Graphene-supported metal oxide monolith

ABSTRACT

A composition comprising at least one graphene-supported metal oxide monolith, said monolith comprising a three-dimensional structure of graphene sheets crosslinked by covalent carbon bonds, wherein the graphene sheets are coated by at least one metal oxide such as iron oxide or titanium oxide. Also provided is an electrode comprising the aforementioned graphene-supported metal oxide monolith, wherein the electrode can be substantially free of any carbon-black and substantially free of any binder.

RELATED APPLICATIONS

This application claims priority to U.S. provisional application Ser.No. 61/745,522 filed Dec. 21, 2012, which is hereby incorporated byreference in its entirety for all purposes.

FEDERAL FUNDING STATEMENT

The United States Government has rights in the invention pursuant toContract No. DE-AC52-07NA27344 between the U.S. Department of Energy andLawrence Livermore National Security, LLC, for the operation of LawrenceLivermore National Laboratory.

BACKGROUND

Iron is the fourth most abundant element (˜5%) in the Earth's crust andthe least expensive among all transition metals (which is ˜100 timescheaper than minor metals such as cobalt). In its oxide form, ironoxides (e.g., FeO, Fe₂O₃, Fe₃O₄) are green materials with littleenvironmental impact and have been investigated as potential anodematerials for high-performance lithium-ion batteries, due largely totheir attractive specific capacity. Fe₂O₃ (alpha-Hematite orgamma-Maghemite), for example, carries a theoretical capacity of 1005mAh/g that is about ˜3 times higher than commercial anode graphite (˜372mAh/g), and is among the highest in various transition metal oxides(e.g., TiO₂, V₂O₅, Cr₂O₃, Mn₃O₄, MoO₂, CO₃O₄, NiO, CuO) (see, forexample, FIG. 7). The combined traits of low-cost, nontoxic,corrosion-resistant, and facile synthesis have evidently made Fe₂O₃ oneof the top candidates as anode materials for lithium-ion batteries.Other metal oxides also are of interest.

Unfortunately, some known hurdles need to be overcome before metaloxides can become the components in lithium-ion batteries, including (1)low electrical conductivity of metal oxides (FIG. 7), which curbs therate performance; (2) defoliation and pulverization of active materialsdue to the large volume expansion, leading to capacity fading and lowcycle life; and (3) scalability, which is desirable for any syntheticapproach in order to have practical applications, as the thickness ofcommercial battery electrodes is typically ˜100-200 μm. Variousscientific strategies have hitherto been actively pursued and become avoluminous subject of lithium-ion batteries.

Among various approaches, graphene/metal oxides as anode materials havebeen under intensive investigations, spurred not only by the highspecific capacities of metal oxides (see FIG. 7A), but also by the highelectrical conductivity, chemical stability, and mechanical robustnessof graphene sheets. To date, a number of composite approaches have beendeveloped, including graphene-anchored, -wrapped, -encapsulated,-layered/sandwiched, or -mixed with metal oxide nanoparticles. Sometimesreduced graphene oxides are used.

Despite high gravimetric energy density and discharge/charge rates oftenwitnessed in some of these composites, most approaches adopt simpledispersion or mixture of graphene with metal oxides, leading to certainshortcomings. First, many strategies only work well when the electrodeis very thin. The short diffusion pathway of nanoparticles cannot betaken advantage of when the anode becomes thicker (>100-200 μm forcommercial applications), as Li⁺ has to diffuse through the thickness ofthe electrode during charge-discharge. This disadvantage inevitablylimits commercialization potential. Second, the majority of electrodesare not carbon-black-free or binder-free. Despite the high electricalconductivity of single sheet graphene, carbonaceous species and/orpolymeric binders are required in most of these approaches. These extrafillers increase electrode weight but contribute little to the lithiumstorage, reducing the overall energy density. In addition, carbonadditives could cause pseudocapacitive behavior in the low-voltage cyclerange that could undermine the role of graphene. Third, the lack ofcontrol in microstructure homogeneity and interface structures, whichprevents in-depth understanding of graphene/nanoparticle interactionmechanisms. The addition of conductive carbons or polymer bindersfurther clouds such studies.

Because of above reasons, the performance characteristics of manyexisting graphene/metal oxides cannot easily scale up with the thicknessof the electrode. Novel architecture designs are needed in order tosolve these and other challenging issues.

SUMMARY

Embodiments provided herein include compositions, devices, and articles,as well as methods of making and methods of using the compositions,devices, and articles.

One embodiment provides, for example, a composition comprising at leastone graphene-supported metal oxide monolith, said monolith comprising(i) a three-dimensional network of graphene sheets crosslinked bycovalent carbon bonds, and (ii) at least one metal oxide embedded insidesaid three-dimensional network.

Another embodiment provides a method comprising: providing a grapheneaerogel monolith; immersing said graphene aerogel monolith in a solutioncomprising at least one metal salt to form a mixture; curing saidmixture to obtain a gel; optionally, heating said gel to obtain agraphene-supported metal oxide monolith. In one embodiment, the heatingstep is not optional but carried out.

Another embodiment provides a method, comprising: providing a porousgraphene aerogel monolith; and depositing at least one metal oxidewithin the porous graphene aerogel monolith by atomic layer depositionto obtain a graphene-supported metal oxide monolith.

Another embodiment provides a device comprising at least onegraphene-supported metal oxide monolith, said monolith comprising athree-dimensional structure of graphene sheets crosslinked by covalentcarbon bonds, wherein the graphene sheets are coated by at least onemetal oxide.

At least one advantage for at least one embodiment is described in thefollowing embodiments including, for example, specific capacities whichare thickness independent, high surface area, high electricalconductivity, and mechanical robustness, and combinations thereof.

BRIEF SUMMARY OF THE FIGURES

FIG. 1 shows a schematic illustration of the synthetic procedures for 3Dgraphene macroassembly (GMA)/Fe₂O₃ hybrids. Step 1, three-dimensionalGMA was fabricated by the reduction of graphene oxides using NH₄OH andsupercritically dried and annealed at 1050° C. Step 2, the FeOOHnanoparticles were anchored inside 3D GMA by a sol-gel method, followedby additional supercritically-drying and annealing at 515° C. The lowerleft drawing shows the graphene-tented Fe₂O₃ structure formed by thisapproach.

FIG. 2 shows microstructures of the as-synthesized graphenemacroassembly (GMA) and graphene/Fe₂O₃ hybrids. (A) A TEM image of GMA.(B) and (C), SEM and TEM images of graphene/Fe₂O₃ hybrids, respectively.(D) Particle size distribution histogram of Fe₂O₃, as measured from atotal count of 169 nanoparticles using a series of TEM images similar tothe one shown in (C). (E) The selected area diffraction pattern of thegraphene/Fe₂O₃. The camera length is 520 mm. (F) A high-resolution TEMimage of graphene/Fe₂O₃, with Fe₂O₃ nanoparticles highlighted insidesquares.

FIG. 3 shows specific capacity, rate performance, and cycle stability ofgraphene/Fe₂O₃ hybrids. The observed specific capacities of (A)graphene/Fe₂O₃, and (B) pure GMA at various discharge/charge currentdensities.

FIG. 4 shows selected discharge/charge cycles for (A) pure GMA and (C)graphene/Fe₂O₃, and their corresponding differential capacity curves,(B) and (D), respectively.

FIG. 5 shows surface morphologies of GMA and graphene/Fe₂O₃ after 30cycles of electrochemical testing. (A) and (B), cross-section SEM imagesof GMA, and (C) and (D), of graphene/Fe₂O₃. Samples in (A) and (C) werecharged to 3V, whereas those in (B) and (D) were fully discharged.

FIG. 6 shows microstructures of graphene/Fe₂O₃ after 30 cycles and inthe lithiation state. (A) A low-magnification TEM image ofgraphene/Fe₂O₃. Some selected particles with back-and-white contrast arehighlighted inside circles. (B) A zoomed-in TEM image of graphene/Fe₂O₃after lithiation. The areas representing Li₂O and Fe nanoparticles arepointed with different white arrows. The size of Fe metal particle isobserved smaller than 5 nm, surrounded by semi-amorphous Li₂O phase. (C)A high-resolution TEM image of isolated Fe particles. The insetfast-Fourier transformation (FFT) pattern is obtained from two squareareas, which exhibit the same FFT pattern.

FIG. 7. (A) A summary of theoretical specific capacities (C), electricalconductivities (σ), and densities (ρ) of different transition metaloxides as anode candidates for lithium-ion batteries. The order of metaloxides is sorted according to their specific capacities, from high tolow. (B) A summary of information and electrochemical performances ofgraphene/Fe₂O₃, as described herein, as anodes for lithium-ionbatteries.

FIG. 8 shows x-ray diffraction data for titanium oxide graphenematerial. Anatase phase with grain size of ˜3 nm was obtained afterannealing at 600° C. for 1 hour under N₂ flow.

FIG. 9 shows Raman spectra for titanium oxide graphene material. Ramanpeaks for titania became stronger after annealing at 600° C. for 1 hourunder N₂ flow.

FIG. 10 shows electrochemical measurements for titanium oxide graphenematerial.

FIG. 11 shows more electrochemical measurements for titanium oxidegraphene material (annealed at 600° C. for 1 hour under N₂ flow).

FIG. 12. (A) Nitrogen adsorption/desorption isotherms and (B) pore sizedistribution for graphene/Fe₂O₃ hybrids.

FIG. 13. Raman spectra of the 3D graphene and a graphene/Fe₂O₃ hybridsample (containing both α- and γ-Fe₂O₃).

FIG. 14. XRD patterns of two representative graphene/Fe₂O₃ samples. Onecontains α- and γ-Fe₂O₃ phases, and the other has 100% γ-Fe₂O₃.

FIG. 15. Differential capacity plots of graphene/(α+γ)-Fe₂O₃ (56 wt. %Fe2O3). The convoluted nature of the inset oxidation peaks can be seen.

FIG. 16. Additional TEM image of graphene/(α+γ)-Fe₂O₃ (40 wt. %),indicating the faceted nature of as-grown particles as manifested by therather straight edges in 2D projection.

FIG. 17. A comparison of simulated Li₂O (cubic) diffraction patternsalong <100> zone axis with the FFT pattern (inset) from our experiments.A good match is found. The simulation was performed using the onlineversion of WebEMAPS software.

FIG. 18. A bright-field TEM image of the lithiated graphene/(α+γ)-Fe₂O₃(40 wt. %) after 30 cycles, showing a partially lithiated particle.

FIG. 19. Microstructures of graphene/(α+γ)-Fe₂O₃ (40 wt. %) after 30cycles. The final state of the sample is charged to 3V (i.e.,delithiated). (A) A TEM image of graphene/Fe₂O₃, showing the facetedcharacteristics of particles. (B) A high-resolution TEM of a selectedparticle. (C) The particle size distribution from the count of 101particles. The fitting by lognormal distribution yields an averageparticle size of 12.7+/−7.0 nm.

FIG. 20. Selected area diffraction (SAD) pattern taken from agraphene/(α+γ)-Fe₂O₃ (40 wt. %) sample after 30 cycles and charged tothe delithiated state (3.0V). The as-synthesized sample contains both α-and γ-Fe₂O₃.

FIG. 21. Bright-field TEM images of graphene/γ-Fe₂O₃ (56 wt. %) in (A)the as-synthesized state, and (B) the lithiated state after 5 cycles.The particle sizes reported in FIG. 7B are measured from a series of TEMimages similar to the ones shown above. The average particle sizes forthe as-synthesized γ-Fe₂O₃ and the lithiated state are 8.1+/−1.2 nm(from 100 counts) and 20.6+/−7.5 nm (from 134 counts), respectively.

FIG. 22. The calculated ratio change (h′/a′) after the lithiation, as afunction of the initial particle shape defined by h/a (see Spherical-capmodel section for the details). The blue dashed line denotes(h′/a′)/(h/a)=1 (i.e., uniform expansion line). Note the substantialreduction of h′/a′ after the lithiation, suggestive of “pancake”shape-change behavior. The calculation parameters are 2a=8.1 nm, and2a′=20.6 nm, both of which are measured by TEM (see FIG. 7B, γ-Fe₂O₃ andFIG. 21). The volume expansion is assumed as ˜96%.

FIG. 23 shows specific capacity, rate performance, and cycle stabilityof graphene/Fe₂O₃ hybrids. The observed specific capacities of (A)graphene/Fe₂O₃, and (B) pure GMA at various discharge/charge currentdensities.

FIG. 24 shows selected discharge/charge cycles for (A) pure GMA and (C)graphene/Fe₂O₃, and their corresponding differential capacity curves,(B) and (D), respectively.

FIG. 25. (a) Schematics of solid-electrolyte-interphase (SEI) formationon graphene surface upon discharge and its effect on suppressing surfacereactions that contribute to reversible Li storage capacity. (b)Schematics of a proposed, possible mechanism for additional reversiblecapacity that is enabled by Fe₂O₃ particle expansion/shrinkage on top ofthe graphene surface.

DETAILED DESCRIPTION

Introduction

References cited herein can be used to practice and better understandthe claimed inventions and are incorporated by reference herein in theirentireties for all purposes.

Priority U.S. provisional application Ser. No. 61/745,522 filed Dec. 21,2012 is hereby incorporated by reference in its entirety for allpurposes including working examples and claims.

US Patent Publication 2012/0034442 to Worsley et al., “MechanicallyStiff, Electrically Conductive Composites of Polymers and CarbonNanotubes” is incorporated herein by reference in its entirety.

The article, “Mechanically Robust 3D Graphene Macroassembly with HighSurface Area,” Worsley et al., Chem. Commun., 2012, 48, 8428-8430, isincorporated herein by reference in its entirety.

The article, “Synthesis of Graphene Aerogel with High ElectricalConductivity,” Worsley et al., J. Am. Chem. Soc., 2011, 2, 921-925, isincorporated herein by reference in its entirety.

The article, “High Surface Area, sp2-Cross-Linked Three-DimensionalGraphene Monolith,” Worsley et al., J. Phys. Chem. Letter, 2010,132(40), 14067-14069, is incorporated herein by reference in itsentirety.

The article, “Advanced Carbon Aerogels for Energy Applications,” Bieneret al., Energy & Environmental Science, 2011, 4, 656-667, isincorporated herein by reference in its entirety.

The article, “Carbon scaffolds for stiff and highly conductivemonolithic oxide-carbon nanotube composites,” Worsley et al, Chemistryof Materials, 2011, 23 (12), 3054, is incorporated herein by referencein its entirety.

Graphene-Supported Metal Oxide Monolith

A graphene-supported metal oxide can be a monolith that is mechanicallyrobust, electrically conductive, and of high-surface area. Monolith is aterm known in the art. Monolith and methods for making monolith aredisclosed in, for example, U.S. Pat. No. 5,207,814, U.S. Pat. No.5,885,953, U.S. Pat. No. 5,879,744, U.S. Pat. No. 7,378,188, U.S. Pat.No. 7,410,718, and U.S. Pat. No. 7,811,711.

The graphene-supported metal oxide monolith can comprise, for example,(i) a three-dimensional network of graphene sheets crosslinked bychemical linkage such as covalent carbon-carbon bond, and (ii) at leastone metal oxide embedded in said three-dimensional network. The metaloxide can be in particle form or non-particle form.

The graphene-supported metal oxide monolith can have a thickness of, forexample, 100 μm or more, or 200 μm or more, or 250 μm or more, or about100-1000 μm, or about 200-500 μm.

The graphene-supported metal oxide monolith can be, for example,mesoporous. The average pore size can be, for example, less than 100 nm,or less than 50 nm.

The metal oxide can include, for example, one or more of Fe₂O₃, TiO₂,MnO₂, Mn₃O₄, Fe₃O₄, Co₃O₄, MoO₂, NiO, CoO, CuO, and V₂O₅. Metal oxidescan also be used with Mn, Fe, Co, Ni, Cu, Zn, Zr. Also used can be SnO₂,CO₃O₄, V₂O₅, NiCo₂O₄, NiO₂, Al₂O₃ and SiO₂. Combinations of metal oxidescan be used.

In some embodiments, the metal oxide is selected from Fe₂O₃, TiO₂, SnO₂,NiO_(x), NiCo₂O_(x), CoO_(x), MnO_(x), Al₂O₃, SiO₂, and V₂O₅.

The metal oxide particle can be, for example, a nanoparticle. Theaverage diameter of the metal oxide particle can be, for example, 1-100nm, or 2-50 nm, or 5-20 nm. In one particular embodiment, the metaloxide particle is a Fe₂O₃ nanoparticle. In another particularembodiment, the metal oxide particle is a TiO₂ nanoparticle.

The weight percentage of metal oxide in the graphene-supported metaloxide can be, for example, 5-95%, or 10-90%, or 20-80%, or 30-60%.

The surface area of the graphene-supported metal oxide can be, forexample, of 200 m²/g or more, or 500 m²/g or more, or 700 m²/g or more,or 200-1500 m²/g, or 500-1000 m²/g.

In one embodiment, 50% or more, or 70% or more, or 90% or more of thecrosslinking covalent bonds of the three-dimensional network of graphenesheets are sp² bonds.

In one embodiment, the graphene-supported metal oxide monolith issubstantially free of graphene sheets interconnected only by physicalcrosslinks (e.g. Van der Waals forces). In another embodiment, less than20%, or less than 10%, or less than 5%, or less than 1% of the graphenesheets are interconnected only by physical crosslinks.

In one embodiment, the graphene-supported metal oxide monolith issubstantially free of graphene sheets interconnected only by metalcrosslinks (e.g., noble metal such as Pd). In another embodiment, lessthan 20%, or less than 10%, or less than 5%, or less than 1% of thegraphene sheets are interconnected only by metal crosslinks.

In one embodiment, the graphene-supported metal oxide monolith issubstantial free of graphene sheets with hydroxyl or epoxidefunctionalities. In another embodiment, 5% or less, or 3% or less, or 1%or less, or 0.5% or less, or 0.1% or less of the carbon atoms in thegraphene-supported metal oxide monolith are connected to a hydroxyl orepoxide functionality.

The graphene sheets can be randomly oriented. The graphene sheets canhave lateral dimensions of 100 nm or more, 200 nm or more, or 500 nm ormore. The surfaces of the graphene sheets can be substantially free ofnanoparticles.

In a preferred embodiment, the three-dimensional network of graphenesheets is not made by stacking non-organic material, such as metals,between graphene sheets.

Process for Making Graphene-Supported Metal Oxide Monolith

The graphene-supported metal oxide monolith described herein can beprepared by, for example: (i) providing a graphene aerogel monolith;(ii) immersing said graphene aerogel monolith in a solution comprisingat least one metal salt to form a mixture; (iii) curing said mixture toobtain a gel; (iv) optionally, heating said gel to obtain agraphene-supported metal oxide monolith. In one embodiment, thegraphene-supported metal oxide monolith was obtained without heating. Inone embodiment, the heating step is carried out to obtain thegraphene-supported metal oxide monolith.

The fabrication of graphene aerogel monoliths are disclosed in US2012/0034442 and Worsley et al., Chem. Commun., 2012, 48, 8428-8430,both of which are incorporated herein by reference in its entirety.

The solution for immersing the graphene aerogel can comprise, forexample, water and/or at least one organic solvent. The organic solventcan include, for example, an alcohol such as methanol, ethanol,propanol, and the like.

The solution for immersing the graphene aerogel can comprise, forexample, at least one initiator for the sol-gel reaction. The initiatorcan be, for example, propylene oxide, trimethylene oxide, dimethyleneoxide, and the like.

The metal salt in the solution can comprise, for example, an iron salt,a titanium salt, a manganese salt, a cobalt salt, a molybdenum salt, anickel salt, a copper salt, and/or a vanadium salt. In one particularembodiment, the metal salt comprises iron nitrate and/or iron chloride.In another particular embodiment, the metal salt comprise titaniumalkoxide.

The concentration of the metal salt in the solution can be, for example,0.02-10 M, or 0.05-5 M, or 0.1-2M. In addition, the molar ratio of theinitiator to the metal salt can be tuned to promote nanoparticlenucleation and anchoring on the surface of graphene sheets. The molarratio of the initiator to the metal salt can be, for example, 50:1 to1:1, or 20:1 to 5:1. In a particular embodiment, the metal salt is aniron salt, and the molar ratio of the initiator to the iron salt isabout 11:1.

The mixture comprising the graphene aerogel monolith immersed in thesolution is subjected to sol-gel reaction. After gelation, the gel isheated to crystallize the metal oxide particle. The gel can be heatedat, for example, 200° C. or more, or 250° C. or more, or 300° C. ormore, or 350° C. or more, or 400° C. or more, or 450° C. or more, or500° C. or more, or about 200-800° C., or about 300-600° C. The gel canbe heated for, for example, 1-10 hours, or 2-8 hours, or 3-6 hours.

The graphene aerogel monolith can be prepared by, for example: (i)preparing a reaction mixture comprising a graphene precursor suspensionand at least one catalyst; (ii) curing the reaction mixture to produce awet gel; (iii) drying the wet gel to produce a dry gel; and (iv)pyrolyzing the dry gel to produce the graphene aerogel.

Precursors to graphene are known in the art. For example, graphene oxideis a general term for oxidized graphene, which can be a precursor tographene. Closely related precursors can include, for example, graphiteoxide, single layer graphene oxide, exfoliated graphite, and the like.

In one embodiment, the reaction mixture comprises a graphene oxide (GO)suspension. Methods for making GO are known in the art and disclosed in,for example, Hummer, J. Am. Chem. Soc., 80:1339 (1958), which isincorporated herein by reference in its entirety. In one embodiment, theGO suspension is an aqueous suspension. In another embodiment, the GOsuspension is a suspension of at least one organic solvents, such asalcohol, dimethylformamide, tetrahydrofuran, ethylene glycol,N-methylpyrrolidone, etc. In one embodiment, the GO suspension is anaqueous suspension made by sonicating GO in deionized water. The timefor sonication can range from 0.5-24 hours. The concentration of GO inthe reaction mixture can be 0.1 mg/cc or more, or 1 mg/cc or more, or 2mg/cc or more, or 5 mg/cc or more, or 10 mg/cc or more.

The reaction mixture may also comprise additional reactant known for solgel reactions, though it is may not be necessary for gelation. In oneembodiment, the reaction mixture comprises resorcinol-formaldehyde (RF),phloroglucinol-formaldehyde, phenol-formaldehyde, cresol-formaldehyde,or phenol-furfuryl alcohol. In one embodiment, the reaction mixture isfree of RF. In another embodiment, the reaction mixture comprises RF. Ina preferred embodiment, the GO-to-RF ratio is 0.1 or more.

The reaction mixture also can comprise at least one sol gel catalyst. Inone embodiment, the catalyst is an acid catalyst. In another embodiment,the catalyst is a base catalyst. Catalysts suitable for making grapheneaerogels include, but are not limited to, nitric acid, acetic acid,ascorbic acid, hydrochloric acid, sulfuric acid, sodium carbonate,sodium hydroxide, ammonium hydroxide, and calcium sulfate. Thereactant-to-catalyst ratio may range from 10 to greater than 1000.

In one embodiment, the reaction mixture is cured at a temperature of25-100° C. to produce a wet gel. In another embodiment, the reactionmixture is cured for 4-168 hours at a temperature of 85° C. In a furtherembodiment, the reaction mixture is cured at atmospheric pressure.

In one embodiment, the wet gel is subjected to solvent exchange toremove reaction by-products. Suitable solvent include, but are notlimited to, DI water. In another embodiment, the wet gel is subjected tosolvent exchange to remove water. Suitable solvents include, but are notlimited to, acetone.

In one embodiment, the wet gel is dried in a supercritical gas toproduce a dry gel. Suitable supercritical gases include, but are notlimited to, supercritical CO₂. In another embodiment, the wet gel isdried under ambient temperature and pressure for an extended time suchas at least 24 hours.

In one embodiment, the dry gel is pyrolyzed in an inert gas to produce agraphene aerogel. Suitable inert gases include, but are not limited to,N₂. The drying temperature can be 500° C. or more, or 800° C. or more,or 1000° C. or more.

In one embodiment, a graphene aerogel can be further activated toproduce an activated aerogel with increased surface area. In oneembodiment, the graphene aerogel is thermally activated in an oxidizingatmosphere. Suitable oxidizing atmospheres include, but are not limitedto, CO₂. The temperature for the thermal activation can be 900° C. ormore, or 1000° C. or more.

In an alternative embodiment, the graphene-supported metal oxidemonolith is prepared by: (i) providing a porous graphene aerogelmonolith; and (ii) depositing at least one metal oxide within the porousgraphene aerogel monolith by atomic layer deposition to obtain agraphene-supported metal oxide monolith.

Devices Comprising Graphene-Supported Metal Oxide

The graphene-supported metal oxide monolith described herein can be usedin a variety of devices. For example, it can be used in electrodes,batteries, capacitors, supercapacitors, sensors, actuators, membranes,catalyst supports, and hydrogen storage devices.

In one particular embodiment, the graphene-supported metal oxidemonolith described herein is used in a lithium battery electrode. Theelectrode can be, for example, substantially free of any carbon-black.The electrode can be, for example, substantially free of any carbonadditives. The electrode can be, for example, substantially free of anybinders, such as polymer binders. The electrode can be, for example,substantially free of metal current collectors. The electrode can have athickness of, for example, 100 μm or more, or 200 μm or more, or 250 μmor more, or 100-1000 μm, or 200-500 μm.

In additional to the electrode comprising the graphene-supported metaloxide monolith, the device can further comprise, for example, at leastone counter electrode, at least one electrolyte, at least one separator,and/or at least one current collector. The electrolyte can be a lithiumsalt, and the device can be a lithium ion battery.

Where the metal oxide is Fe₂O₃, the lithium ion battery can have adischarge capacity of, for example, at least 500 mAh/g, or at least 800mAh/g, or at least 1000 mAh/g at 100 mA/g after 30 cycles. The lithiumion battery can have a reversible capacity of, for example, at least atleast 500 mAh/g, or at least 700 mAh/g, or at least 900 mAh/g at 141mA/g (0.14C). The lithium ion battery can have a reversible capacity of,for example, at least at least 400 mAh/g, or at least 500 mAh/g, or atleast 600 mAh/g at 503 mA/g (0.5C).

Where the metal oxide is TiO₂, the lithium ion battery can have areversible capacity of, for example, at least at least 50 mAh/g, or atleast 100 mAh/g, or at least 200 mAh/g at 168 mA/g (1C). Long-term cyclestability testing can show longer life. Fast charging can be alsoachieved.

Graphene Monolith Comprising Lithiated Metal Oxide

Further embodiments of the monolith described herein comprises (i) atleast one graphene-based monolith comprising a three-dimensional networkof graphene sheets crosslinked by covalent carbon bonds, wherein saidthree-dimensional network of graphene sheets defines a surface area,(ii) a first region on said surface area comprising at least onelithiated metal oxide, wherein the first region is not covered by SEI,and (iii) a second region on said surface area covered by SEI.

The surface area covered by the first region can be, for example, atleast 10%, or at least 20%, or at least 30%, or at least 40%. Thesurface area covered by the second region can be, for example, less than90%, or less than 80%, or least than 70%, or less than 60%, or less than50.

In some embodiments, the metal oxide is selected from Fe₂O₃, TiO₂, SnO₂,NiO_(x), NiCo₂O_(x), CoO_(x), MnO_(x), Al₂O₃, SiO₂, and V₂O₅.

Additional embodiments are provided in the following non-limitingworking examples.

WORKING EXAMPLES

Additional embodiments are also provided in the following non-limitingworking examples. For example, graphene-supported metal oxide monolithswere prepared and characterized.

Example 1 Iron Oxide/Graphene Monolith

Material Synthesis

Thick (˜250 μm), carbon-black- and binder-free, nanoporousgraphene/Fe₂O₃ hybrids were synthesized through a chemical sol-gelmethod. In contrast to previous approaches (and in order to achievethick and binder free electrodes), the synthetic strategy began with anovel 3-dimensional (3D) graphene macroassembly (GMA) scaffold, FIG. 1(Step 1), using the method reported in Worsley et al., Chem. Commun.2012, 48 (67), 8428-8430. The as-synthesized GMA has a thickness of ˜250μm, a density of ˜70 mg/cm³, a very high surface area up to ˜1500 m²/g,high electrical conductivity (˜2 S/cm), and nanometer-sized open porousdistributions, a bulk part of which is smaller than 10 nm. Both thedensity and surface area of the GMA are higher than most 3D grapheneassemblies reported in the literature (see Chen et al., Nature Mater.2011, 10 (6), 424-428). The GMA is also mechanically robust, with a highcompressive failure strain of 80-100%, making it suitable for electrodeapplications.

After GMA synthesis, the 3D GMA was immersed in an ethanolic solution ofFe (III) salt (e.g. iron nitrate, iron chloride) to which an initiator(e.g. propylene oxide, trimethylene oxide) was added (i.e., Step 2 inFIG. 1) (see Gash et al., Chem. Mater. 2003, 15 (17), 3268-3275). Ironsalt concentrations ranged from 0.175 to 1.4 M. As previously reported(Worsley et al., Chem. Mater. 2011, 23 (12), 3054-3061; Worsley et al.,J. Mater. Chem. 2009, 19 (31), 5503-5506), preferential nucleation ofthe nanoparticles on high surface area carbon can be achieved when thesol-gel chemistry is appropriately tuned. In this case, the molar ratioof initiator-to-Fe was set to 11:1 in order to promote nanoparticlenucleation and anchoring of FeOOH nanoparticles on the surface ofgraphene sheets, instead of in solution. After the particles wereformed, the coated assembly was fired at 515° C. under nitrogen for 3hours to convert FeOOH to Fe₂O₃.

Characterization and Analysis

The microstructures of the as-synthesized pure GMA and graphene/Fe₂O₃hybrids are exhibited in FIG. 2. For pure GMA, a transmission electronmicroscopic (TEM) image shown in FIG. 2A indicates that it is mostlycomprised of single-layer graphene sheets; but stacked layers arevisible with a measurable interlayer spacing of ˜0.385 nm. Ramanspectroscopy characterizations reveals that the as-synthesized 3D GMAhas a D/G band ratio of 1.37. After Fe₂O₃ deposition, scanning electionmicroscopy (SEM) studies (FIG. 2B) confirmed that nanoparticlesuniformly embedded among 3D graphene networks. The existence of Fe₂O₃was further confirmed by Raman spectroscopy, which shows that theas-synthesized 3D GMA has a D/G band ratio of 1.37 (FIG. 12).

At 40% Fe₂O₃ weight load, the Brunauer-Emmett-Teller (BET) measurementsusing nitrogen isothermal adsorption indicate that the hybrid has aspecific surface area of ˜680 m²/g, which is higher than those reportedin the literature (Zhou et al., Chem. Mater. 2010, 22 (18), 5306-5313).In addition, all pores inside the hybrids are smaller than 100 nm, asdetermined by Barrett-Joyner-Halenda (BJH) method.

Further, at 56% Fe₂O₃ weight load, the BET measurements using nitrogenisothermal adsorption indicate that the hybrid has a specific surfacearea of ˜700 m²/g (FIG. 13A). In addition, the bulk part of open poresinside the hybrids is smaller than 10 nm (FIG. 13B), as determined byBJH method. These open nanopores are expected to not only serve astransport channels for electrolyte, but also offer ample room for thevolume expansion of metal oxides during the lithiation. As such, thesenanoporous structures are one of the key reasons why our electrodes canbe ultrathick.

The representative TEM image in FIG. 2C illustrates that the sizes ofmost Fe₂O₃ nanoparticles are well below 50 nm, as can be better revealedin the statistical particle distribution histogram (FIG. 2D), whichindicates an average particle size of ˜12.5±5.5 nm. The nanoporouscharacteristics of GMA are also clearly visible under TEM. The selectedarea diffraction (SAD) pattern from TEM in FIG. 2E shows a mixture phaseof α- and γ-Fe₂O₃, consistent with the x-ray diffraction results (FIG.14). The high-resolution TEM investigations, one example of which isshown in FIG. 2F, indicate the good crystalline quality ofas-synthesized nanoparticles and tented nature of Fe₂O₃ with most edgesof the nanoparticles in tight bond with graphene sheets. Thesegraphene-tented structures not only offer great electron pathway to thenanoparticles during lithiation-delithiation processes, but also alloweasy volume expansion and contraction for Fe₂O₃ nanoparticles withouthaving to break electrical contacts.

Electrochemical Performance of Graphene/Fe₂O₃-40 wt. % Loading

The electrochemical performance of the graphene/Fe₂O₃ as anodes wascharacterized using a half-cell configuration, with Fe₂O₃ weightpercentages at 40%. Note that the as-synthesized freestanding films weredirectly used without any carbon additives, or polymer binders, or metalcurrent collectors. Despite the very large thickness (˜250 μm) of theelectrodes, graphene/Fe₂O₃ hybrids showed excellent electrochemicalperformance, the cycle stability and rate performance of which areillustrated in FIG. 3A. At a current density of 141 mAh/g (i.e., 0.14C,where C=1005 mAh/g), the graphene/Fe₂O₃ sample exhibited a dischargecapacity of ˜2020 mAh/g (equivalent to 12.1 Li⁺ uptake per Fe₂O₃molecule based on the total weight) and a charge capacity of ˜818 mAh/gat the first cycle. The relatively low charge capacity of the sample inthe first cycle is likely due to the poor electrolyte wetting, as theanode did not contain any polymer binders and had ultra-thickness. Thisis consistent with the lag response of the specific capacity whenever adifferent discharge/charge rate is applied (see FIG. 3A). As the wettingimproved, the charge capacity steadily increased and stabilized after˜10 cycles at ˜922 mAh/g. At a higher current density of 503 mAh/g, thesample had a reversible capacity of ˜656 mAh/g, which remains well abovethe theoretical capacity of graphite and is indicative of good rateperformance. Significantly, the reversible capacity of graphene/Fe₂O₃retained at a value of ˜1166 mAh/g (equivalent to 7.0 Li⁺ based on thetotal weight) at a current density of 100 mAh/g after 30 cycles. Theseresults demonstrated excellent cycle stability of the graphene/Fe₂O₃hybrids. Since Fe₂O₃ has a theoretical Li⁺ intake of 6 per molecule, theadditional reversible capacity (i.e., 1.0 Li⁺ at 100 mAh/g) observed inthe hybrids may be related to the formation of reversible organicgel-like films (Larcher et al., J. Electrochem. Soc. 2003, 150 (12),A1643-A1650) and/or Li⁺ insertion/intercalation into graphene defectsites or interlayers. Notably, the initial uptake of 12.1 Li⁺ per Fe₂O₃molecule is higher than all previously reported values, includinggraphene/Fe₂O₃ and nanosized Fe₂O₃ particles with values of 10.1, 8.8,and 8.6 Li⁺ uptake reported previously (Larcher et al., J. Electrochem.Soc. 2003, 150 (12), A1643-A1650; Morales et al., J. Electrochem. Soc.2005, 152 (9), A1748-A1754). Such a large Li⁺ uptake may be related tothe pseudocapacitive behavior typically associated with large surfacearea electrodes (Zhu et al., ACS Nano 2011, 5 (4), 3333-3338; Larcher etal., J. Electrochem. Soc. 2003, 150 (12), A1643-A1650). Despite the verylarge thickness of electrodes used in the experiments and the lack ofcarbon additives and binders, the reversible capacity of thegraphene/Fe₂O₃ hybrids at low current density (˜100 mAh/g) is higherthan other graphene-based hybrids, whereas at higher cycle rates (>503mAh/g) it is only slightly lower those reported in the literature (Zhuet al., ACS Nano 2011, 5 (4), 3333-3338; Zhou et al., Chem. Mater. 2010,22 (18), 5306-5313). These informative experimental results demonstratethe good rate performance, high cycle stability, and high specificcapacities of the graphene/Fe₂O₃ hybrids.

By comparison, the pure GMA samples (˜250 μm thick, also withoutpolymeric binders or carbon-black) showed unimpressive cycle stabilityand low capacity, FIG. 3B. At a current density of 100 mAh/g, the firstdischarge capacity of GMA achieved a high value of 1077 mAh/g; butdecreased to 270 mAh/g in the following charge process. The Coulombicefficiency of the first cycle was ˜25%. The discharge capacity in thesecond cycle decreased to 310 mAh/g, which became stabilized at thelevel of ˜148 mAh/g after 10 cycles. The reversible capacity seen in theGMA was substantially lower than the theoretical capacity of graphite(˜372 mAh/g). At a higher current density (>503 mAh/g), pure GMA sampleexhibited a capacity that is below ˜60 mAh/g. The unimpressivereversible capacity and poor rate-performance observed in GMA are instark contrast to the large and impressive capacity obtained by 3Dgraphene/Fe₂O₃ electrodes. This underscores a strong synergistic effectbetween Fe₂O₃ nanoparticles and graphene.

To study the mechanistic processes occurring in pure GMA and thehybrids, FIG. 4 shows the discharge/charge curves of several selectedcycles at a current density of 100 mAh/g and their correspondingdifferential capacity curves for GMA (FIGS. 4A and B) and graphene/Fe₂O₃hybrids (FIGS. 4C and D), respectively. During the first discharge ofthe GMA, the voltage profile showed three reduction peaks at 1.07 V,0.69 V and <0.5 V (FIG. 4B), the origin of which could be related to theLi⁺ insertion into graphene defects/edges and/or the decomposition ofelectrolyte and/or the formation of SEI. In the following cycles,however, two high-voltage peaks disappeared, suggesting a nonreversibleprocess associated with these peaks. The last plateau associated with<0.5V remained after 10 cycles and thus may represent reversible lithiumuptake by GMA. Correspondingly, the two oxidation peaks for pure GMAalso disappeared with increasing cycles (inset of FIG. 4B).

For the graphene/Fe₂O₃ sample, FIG. 4D, three main reduction-peaks wereidentified: 1) the weak peaks around ˜1.6 V, which has been attributedto the intercalation of Li⁺ into Fe₂O₃; 2) the strong peaks at 0.6 V-1.0V, likely due to the conversion reaction (i.e., the iron reduction fromFe³⁺ to Fe⁰:Fe₂O₃+6Li⁺+6e

2Fe+3Li₂O  (I);and 3) the peaks below 0.5V, which could be related to the formation oforganic gel-like films or Li⁺ insertion into GMA. During the chargeprocess, the hybrid sample displayed four oxidation peaks at 1.2V, 1.5V,1.8V, and 2.4V. While the middle two peaks (i.e., O₂ and O₃) have beenattributed to the step oxidations of iron from Fe⁰ to Fe³⁺ by lithiumexaction, the first (O₁) and last (O₄) peaks are likely linked to GMA astheir positions are similar to the oxidation peaks of the pure graphenesamples. Unlike GMA, however, these two oxidation peaks in the hybridsremained observable even after 10 cycles; i.e., they became reversibledue to the presence of Fe₂O₃/Fe nanoparticles—an intriguing phenomenonthat might be linked to the catalytic ability of nanosized Fe₂O₃/Feparticles, as the oxidation peaks were also observed to shift downwardcompared to those of pure GMA, indicative of easier delithiationprocesses for graphene inside the hybrids. Although the potentialcatalytic activities of metal nanoparticles in promoting thedecomposition of electrolyte and the formation of gel-like films weresuggested previously (Zhou et al., Chem. Mater. 2010, 22 (18),5306-5313; Morales et al., J. Electrochem. Soc. 2005, 152 (9),A1748-A1754), it has not been reported that such catalytic activitiesmay also help lithium ion interactions with graphene. This hypothesis isbased largely on the earlier experimental documentations that gel-likefilms can only offer pseudocapacitive behavior that is not consistentwith the reappearance of the oxidation peaks for graphene in thesynthesized graphene/Fe₂O₃ materials.Electrochemical Performance of Graphene/Fe₂O₃-56 wt. % Loading

The electrochemical performance of several graphene/Fe2O3 samples (withFe₂O₃ load ratio of 456 wt. %) as anodes is investigated using ahalf-cell configuration. Note that in all the cases, the as-synthesizedfreestanding films are directly used without any carbon additives, orpolymer binders, or metal current collectors. Despite the very largethickness (˜250 μm) of our electrodes, graphene/Fe₂O₃ hybrids (56 wt. %Fe₂O₃) show excellent electrochemical performance, with the cyclingstability and rate capability illustrated in FIG. 23A. At a currentdensity of 141 mA/g (i.e., 0.14C if normalized against the Fe₂O₃capacity of 1C=1005 mAh/g), the graphene/Fe₂O₃ sample exhibits adischarge capacity of ˜1633 mAh/g (based on the total weight of theelectrode) and a charge capacity of ˜796 mAh/g at the first cycle. SinceFe₂O₃ has a theoretical Li⁺ intake of 6 per molecule, the large initialuptake of Li⁺ observed in the graphene/Fe₂O₃ may be related to Li⁺binding to graphene surface sites (including structural defects andimpurity such as residual oxygen), Li⁺ intercalation into graphenelayers, the formation of organic gellike films (Laruelle et al., J.Electrochem. Soc. 2002, 149 (5), A627-A634), and/or other irreversibleproducts such as the solid electrolyte interphase (SEI) (Arora et al.,J. Electrochem. Soc. 1998, 145 (10), 3647-3667).

Despite the lack of binders and the very large thickness, we note thatthe reversible capacity of our electrodes stabilizes in less than 5cycles at 790 mAh/g, demonstrating the excellent wetting ability of ourmaterials. This is in contrast with some dense films reported in theliterature (Wang et al., ACS Nano 2010, 4 (3), 15871595). At a highercurrent density of 503 mA/g, the reversible capacity decreases to ˜544mAh/g, which, however, remains well above the theoretical capacity ofgraphite (˜372 mAh/g) and is indicative of good rate performance.Significantly, the reversible capacity of graphene/Fe₂O₃ stabilizes at avalue of ˜853 mAh/g at a current density of 100 mA/g after 30 cycles.Among several representative samples we have investigated so far (FIG.7B), importantly, the reversible capacity of our materials at the lowcurrent density (˜100 mA/g) after 30 cycles either exceeds or is on parwith those reported from other graphene/iron oxides (Zhu et al., ACSNano 2011, 5 (4), 3333-3338; Zhou et al., Chem. Mater. 2010, 22 (18),5306-5313), and even stays at comparably high levels at higher currentdensities (>503 mA/g). These informative experiments demonstrate thegood rate performance, excellent cycling stability, and high specificcapacities of our materials.

In comparison, the pure GMA samples (˜250 μm thick, also without polymerbinders or carbon-black) show severe capacity fading and low reversiblecapacity, FIG. 23B. At a current density of 74.4 mA/g, the firstdischarge capacity of GMA achieves a high value of 1077 mAh/g; butplunges sharply to 270 mAh/g in the following charge process. Thedischarge capacity decreases to 310 mAh/g in the second cycle, andstabilizes at ˜148 mAh/g after 10 cycles, which is in fact substantiallylower than the theoretical capacity of graphite. At a higher currentdensity (>372 mA/g), the GMA sample exhibits a stable capacity below ˜60mAh/g. Such a low reversible capacity and poor rate performance observedin GMA are in stark contrast to the large and impressive capacityobtained in 3D graphene/Fe₂O₃ electrodes. This underscores the strongsynergistic interplays between Fe₂O₃ nanoparticles and graphene.

To survey the mechanistic processes occurring in pure GMA and thehybrids, FIG. 24 shows the charge/discharge curves of several selectedcycles at a current density of 100 mA/g and their correspondingdifferential capacity curves for GMA (FIGS. 24A and 24B) andgraphene/Fe₂O₃ hybrids (FIGS. 24C and 24D), respectively. During thefirst discharge of the GMA, the voltage profile in the differentialcapacity curves shows three reduction peaks at 1.07 V (R1), 0.69 V (R2)and <0.5 V (R3), FIG. 24B. The origin of these peaks is currently notwell understood; but might be related to the Li⁺ binding with graphenedefects/edges and/or the decomposition of electrolyte and/or theformation of SEI. In the following cycles, however, two highervoltagereduction peaks (i.e., R1 and R2) disappear, suggesting a nonreversibleprocess associated with these peaks. The last plateau associated with<0.5 V (R3) and the corresponding oxidation peak (O1) (inset of FIG.24B) remain present after 10 cycles and thus may represent reversiblelithium uptake by GMA. Correspondingly, the two oxidation peaks athigher voltages (i.e., O2 and O3) for pure GMA also disappear withincreasing cycles.

For graphene/Fe₂O₃ electrodes, the samples with higher load of Fe₂O₃typically exhibit rather convoluted oxidation peaks that can inprinciple be separated by proper fitting procedures but are visuallychallenging to resolve (FIG. 15). For the clarity of presentation, FIG.24D shows the reduction/oxidation behavior of a sample with 40 wt. %Fe2O3. Three main reduction peaks can be identified: 1) the weak peakaround ˜1.6 V (R1), which has been attributed to the intercalation ofLi⁺ into Fe₂O₃ to form rock salt Lix Fe₂O₃ (x=˜12); 2) a strong peak at0.6 V 1.0 V (r2), likely due to the second step of the main conversionreaction:Fe₂O₃+6Li⁺+6e

2Fe+3Li₂O  (I)(i.e., iron reduction from the intermediate compound to Fe⁰), as well aspossible contribution of the R2 peak (in FIG. 24B) from graphene; and 3)the peaks below 0.5V (R3), which could be related to the formation oforganic gel-like films, Li⁺ insertion into GMA, or irreversible SEIformation. During the charging process, the hybrid sample appears todisplay five oxidation peaks at <0.5 (O1), 1.2V (O2), 1.5V (O3), 1.8V(O4), and 2.4V (O5). While the middle two peaks (i.e., O3 and O4) havebeen attributed to the two step oxidations of iron from Fe⁰ to Fe³⁺ bylithium exaction, the other three peaks (O1, O2, O5) are likely linkedto the GMA as their positions are close to the oxidation peaks of thepure graphene samples (O1, O2, O3, respectively). Unlike the pure GMA,however, two oxidation peaks (O2 and O5) in the hybrids remainobservable even after 10 cycles; i.e., they become more reversible dueto the presence of Fe2O3/Fe Nanoparticles—an intriguing phenomenon thathas not been reported. We speculate that this may be linked to thevolume expansion/contraction of nanoparticles that allows for on-and-offexposure of graphene fresh surface, or related to the catalytic abilityof nanosized Fe₂O₃/Fe particles. We note that the catalytic behavior ofmetal nanoparticles in promoting the decomposition of electrolyte andthe formation of gel-like films have been suggested by earlierexperiments, which archetypically offer pseudocapacitive behavior. Ithas not been proposed/reported that such catalytic activities may alsohelp redox reactions between lithium ions and graphene (defects orimpurities), which could be associated with the recurring oxidationpeaks in our samples.Surface Morphology

The carbon-black- and binder-free nature of the graphene/Fe₂O₃electrodes allows further investigation on the surface morphologyevolution details after multiple charge-discharge cycles (30 cycles inall the cases reported here), without the complicated effects of extrafillers. FIGS. 5A and 5B indicate that the surface of GMA was relativelyclean and smooth in both delithiated (i.e., FIG. 5A) and lithiated(i.e., FIG. 5B) states. The graphene sheets looked slightly thicker andsome small voids appear blocked in FIG. 5B, whereas bubble-like featureswere visible on the surface of graphene in the delithiated state. These“bubbles” were relatively stable under the electron beam used forimaging, which may be related to the reduction products of electrolyte(such as alkyl carbonates); however, continuous gel-like films wereobserved on the graphene surface. In contrast, the gel-like films werevisible for the graphene/Fe₂O₃ sample in the discharged state, FIG. 5C,judged from the rather thick ligaments under SEM. In the charged state,FIG. 5D, it is noticed that much space of the electrode was filled bybulged ligaments, with some voids remaining visible. It is noteworthy tomention that both pure GMA and graphene/Fe₂O₃ remained completely intactafter 30 cycles, as evidenced by the well-preserved porous structuresand ligaments.

To further investigate the atomic scale mechanisms, TEM samples wereprepare after multiple cycles inside the glove box and transferred tothe TEM holder using an argon (99.999+% purity) sealed vial. In order toexpose and better reveal the nanoparticle morphologies, theelectrochemically cycles samples were washed in acetonitrile for morethan 10 hrs. The microstructures of graphene/Fe₂O₃ in both fullylithiated and delithiated states were investigated (after 30 cycles),respectively. Under the lithiated state, FIG. 6A, almost all particlesexhibited oval or spherical shapes, with black-and-white contrastco-existing often within one single nanoparticle. The round geometriesof the nanoparticles were in contrast to the faceted nature of particlesseen in the as-synthesized state (see FIG. 2C and FIG. 16). A zoomed-inTEM image shown in FIG. 6B revealed the existence of pure Fenanoparticle surrounded by Li₂O, as evidenced by the lower contrast ofLi₂O and the crystalline morphology of Fe metal. Under thehigh-resolution TEM, lattice fringes were not readily visible for someLi₂O particles, suggesting the semi-amorphous nature of this phase.These postmortem TEM examinations confirmed the forward reaction ofequation (I). Occasionally, however, crystalline Li₂O particles werefound, two examples of which are shown in FIG. 6C. Both particles wereidentified to have the zone axis of <100>, as confirmed by the insetfast-Fourier transformation (FFT) pattern (i.e., both areas have theidentical FFT patterns) and diffraction pattern simulations (FIG. 17).The existence of these somewhat isolated Li₂O could be related to themethod for preparing the TEM samples. Intriguingly, partially lithiatedFe₂O₃ particles were also located inside the hybrid samples (FIG. 18).This behavior seemed to only occur to some rather large Fe₂O₃ particles(>250 nm). This may suggest that the lithiation/delithiation behavior ofmetal oxides is very sensitive to their sizes and agglomerations—anobservation that supports some earlier reports (Poizot et al., Nature2000, 407 (6803), 496-499). These results accentuate the importance ofusing nanosized metal oxide particles for Li storage, and echo the goodcycle performance of the graphene/Fe₂O₃ sample. It is found that theround shape nanoparticles can be reversed back to the facetednanocrystals after they are fully delithiated, as further confirmed byTEM investigations (FIG. 19). The statistical particle size distributionafter 30 cycles revealed insignificant average size changes (with aslightly larger standard deviation). This could be attributed to theexcellent confinement effect of graphene sheets and the prevalentnanoparticles in the graphene/Fe₂O₃ samples that are more resistant topulverization compared to microsized counterparts.

Intriguingly, TEM studies further reveal the disappearance of α-phasefor Fe₂O₃ after 30 cycles, as indicated by the SAD pattern (FIG. 20),which suggests that only γ-Fe₂O₃ remains. Furthermore, the SAD patternno longer yields bright diffraction spots except for the continuousrings, indicating the disintegration of any preexisting large particlesor the transformation of single-crystalline to polycrystalline Fe₂O₃particles upon long-term cycles. The preference of α-Fe₂O₃transformation to more open cubic structure (i.e., γ-Fe₂O₃) during theLi-ion intercalation has been reported previously (Larcher et al.,Electrochem. Soc. 2003, 150 (12), A1643-A1650; Jain et al., Chem. Mater.2006, 18 (2), 423-434), which was also found not to revert back to thehexagonal α-Fe₂O₃. These earlier observations agree with our TEMinvestigations. This structural irreversibility, however, does not seemdetrimental to the electrochemical reversibility of our hybridmaterials, as we observe similar specific capacity for two comparisonsamples [see FIG. 7B, (α+γ)-Fe₂O₃ vs. γ-Fe₂O₃].

Another important observation from TEM studies is that the averagediameter of nanoparticles is found to increase by more than a factor of2.5 in the lithiated state (FIG. 7B, and FIG. 21). This would haveprojected a volume expansion of over a thousand percentages providedthat nanoparticles were in spherical shape and expanded uniformly duringthe lithiation. We observed this diameter change trend in both sets ofsamples that have been examined under TEM. To understand this apparentparadox of the measured particle sizes and the projected volumeexpansions, we perform empirical analysis using a spherical-cap modelthat is consistent with classical nucleation and growth theory ofparticles on a flat surface. Our calculations, interestingly, suggestthat nanoparticles are likely to change their aspect ratios and “spread”preferentially along the graphene/nanoparticles interfaces (i.e.,anchored interfaces) upon the lithiation, leading to pancakelike shapesthat are supported by our TEM observations. FIG. 6D schematicallyillustrates the nanoparticles expansion process on graphene sheet (alsosee FIG. 22). This surprising particle shape change is probably becausethe conversion reaction, which requires the participation of electrons(Eqn. 1), occurs preferentially on the graphene surface. Becauseelectron transport within metal oxides (Fe₂O₃ and Li₂O) is extremelysluggish due to their poor conductivity, the reaction is kineticallyfavored to take place on the graphene surface, where electrons caneasily reach the reaction front through facile transport within thegraphene layer. In addition, ion species (e.g. Li⁺ and O²) are likely tohave larger mobility along the solid/graphene or liquid/grapheneinterface than in the solid phases, further promoting graphene surfaceas the preferred reaction site. Consequently, the reaction product(Li₂O+Fe) upon lithiation has a larger growth rate along the graphenesheets, which produces a significant change in the aspect ratio of thenanoparticles. Furthermore, the average Fe₂O₃ particle size in thedelithiated state after 30 cycles measured from TEM images showsinsignificant change from the as synthesized state, indicating thereversibility of most particle shape changes.

The finding of the significant shape change of Fe₂O₃ nanoparticles upon(de)lithiation prompts us to propose one possible reason for thesynergistic effect observed in the graphene/Fe₂O₃ hybrid electrodes.After multiple charge/discharge cycles, our graphene/Fe₂O₃ samplesexhibit a reversible capacity that is notably larger than thetheoretical capacity of Fe₂O₃ itself (i.e., 56 wt. % Fe₂O₃ contributes amaximum of ˜563 mAh/g capacity), indicating that the hybrid capacityalso derives from sources other than Fe₂O₃. The presence of multiplepeaks and a large background on the differential capacity curves (FIGS.4C and D) suggests that the extra capacity could have severalcontributions, involving Li⁺ binding to graphene and its structuraldefects, reaction between Li⁺ and impurity (e.g., residual oxygen),and/or the formation of organic gel-like films. While the detailedmechanisms of such auxiliary reactions remain to be clarified, it isplausible that they occur predominantly on the graphene surface becauseof easy access to electrons. However, a remarkable finding of this workis that these possible reactions only contribute significantly toreversible capacity in the presence of Fe₂O₃ nanoparticles. In the pureGMA sample, the discharge capacity is merely ˜148 mAh/g after 10 cyclesat a current density of 100 mA/g and even lower at higher dischargerates.

It is well known that side reactions (e.g. electrolyte decomposition)cause SEI formation on the surface of battery anodes (e.g. graphite,silicon) during the first few cycles (Arora et al., J. Electrochem. Soc.1998, 145 (10), 3647-3667; Peled, E., Lithium stability and filmformation in organic and inorganic electrolytes for lithium batterysystems, Academic Press: New York, 1983; page 43; Huang et al., Science2010, 330 (6010), 1515-1520). The large irreversible capacity (˜700mAh/g) seen in the pure GMA sample upon first discharge may beattributed to the formation of a stable SEI film on graphene surface.This SEI layer could be responsible for the very low reversible capacityin subsequent cycles by passivating the graphene surface and inhibitingthe reversible reactions mentioned above, FIG. 25A. In thegraphene/Fe₂O₃ hybrid electrodes, however, we propose that thepreferential expansion of nanoparticles on graphene sheets uponlithiation plays a critical role in covering the graphene surface toprevent SEI formation on the top while still allowing the reversibleauxiliary reactions to occur forwardly, as illustrated in FIG. 25B. Asthe particles shrink during delithiation, the surface underneath isre-exposed to the electrolyte and becomes delithiated. The graphenesurface around Fe2O3 nanoparticles can thus remain SEIfree upon cyclingand contribute to the reversible capacity. This scenario is consistentwith the persistent appearance of the graphene-related oxidation peaks(O2 and O5) on the differential capacity curves (FIG. 24D).

To estimate the surface area “protected” by the nanoparticles, wemeasured the average particle size of a hybrid sample (56 wt. % Fe₂O₃)consisting of only γ-Fe₂O₃ from TEM images, which is 8.1 nm in thepristine state and 20.6 nm in the lithiated state. Using these data andassuming that the pristine Fe₂O₃ particles have a hemispherical shape, asimple calculation shows that the nanoparticles will cover ˜41% of thetotal graphene surface area in the lithiated state (up from 6% in thedelithiated state). The actual coverage may be even higher consideringthat we use the upper limit of GMA's measured specific surface area(˜1500 m²/g) in the calculation and the graphene sheets in our sampleare highly curved and could make contact with Fe₂O₃ particles onmultiple sides. Therefore, the auxiliary reactions can occur on a largefraction of the graphene surface, which is consistent with thesignificant synergistic capacity enhancement seen in our hybrid samples.The mechanism we propose here may also explain the similar effectpreviously reported for several other graphene/metal oxides systems (Zhuet al., ACS Nano 2011, 5 (4), 3333-3338; Wang et al., J. Am. Chem. Soc.2010, 132 (40), 13978-13980; Wu et al., ACS Nano 2010, 4 (6),3187-3194), although it remains to be confirmed that metal oxideparticles in these samples undergo an analogous shape change uponcycling. A significant and verifiable prediction from our hypothesis isthat decreasing the particle size of metal oxides will lead to largergraphene-related reversible capacity, as the surface area covered bylithiated particles scales inversely with particle diameter.

By taking advantage of the unique mechanical robustness and highelectrical conductance of 3D graphene assembly, we have developed ascalable approach to fabricating additive-free 3D graphene/Fe₂O₃ hybridelectrodes with commercially viable thickness for Li-ion batteries. Asanodes graphene/Fe₂O₃ nanoporous films exhibit a high reversiblecapacity (>850 mAh/g at 100 mA/g), good rate performance and cyclingstability. We observe a strong synergistic effect in the hybrids thatcannot be offered by the simple conversion reactions of metal oxides orgraphene alone. TEM investigations reveal major atomic processes ofconversion reactions for Fe₂O₃ and a size-dependentlithiation/delithiation behavior. These mechanistic studies stress theimportance of employing nanosized particles in retaining high specificcapacity and good rate performance of the hybrid materials. Note thatFe₂O₃ is an insulating material with poor electrical conductivity, whichhas to rely solely on the 3D graphene networks to deliver electrons andoffer mechanical support. Our work thus demonstrates the enabling rolesof 3D graphene in providing a conductive network and maintainingstructural integrity of the electrodes without the need for carbonadditives or polymer binders. Based on the TEM observations, we proposea graphene surface protection mechanism mediated by metal oxide particleshape change to rationalize the synergistic effect, which may shed lighton the pathway towards further optimization of graphene/metal oxideelectrodes. Our synthetic method can be extended to other transitionmetal oxides, and with the availability of relatively cheap graphenematerials, is likely to provide a viable route to the fabrication ofhigh-performance electrodes for LIBs

In summary, the working examples help demonstrate a scalable approach tofabricating commercial thickness electrodes, using 3D graphene assemblywas developed. The carbon-black- and binder-free graphene/Fe₂O₃nanoporous materials as anodes exhibited a high reversible capacity(˜1166 mAh/g at 100 mAh/g current density), good rate performance andcycle stability. A strong synergistic effect in the hybrids was observedthat cannot be offered by the simple conversion reactions of metaloxides or graphene alone. The contributions of graphene to the specificcapacity and rate performance were drastically improved with theexistence of metal oxides. TEM investigations revealed some atomicprocesses of conversion reactions for Fe₂O₃ and a strong size-dependentlithiation/delithiation behavior. The synthetic method described here isexpected to apply to other transition metal oxides as well, and thusprovide a viable route to engineering high-performance electrodes forlithium ion batteries.

Conclusion

Despite a number of reports on graphene-based metal oxide anodematerials, the prior art approaches are either not easily scalable tolarge thickness for electrodes or require carbon additives and/orpolymeric binders. Reported here is an ultra-thick (˜250 μm),carbon-black-free and binder-free 3-dimensional (3D) graphene/Fe₂O₃hybrid architecture, where Fe₂O₃ nanoparticles (˜12.5±5.5 nm) areanchored and tented inside graphene networks. The graphene/Fe₂O₃ hybridsare freestanding films and mechanically robust. As an anode material,graphene/Fe₂O₃ offers a large reversible specific capacity of ˜1166mAh/g at the current density of 100 mAh/g after 30 cycles, and good rateperformance. A strong synergistic behavior between Fe₂O₃ and 3D graphenenetworks was observed that cannot be offered by the mathematic additionof the conversion reaction of Fe₂O₃ and the lithium intercalations intographene. Scanning and transmission electron microscopy after variouselectrochemical cycles revealed the significance of interplay betweenFe₂O₃ and graphene, and strong size-dependent electrochemical behaviorof metal oxides. The synthetic approach demonstrates a viable scale-uproute to constructing graphene/metal oxides as anode materials forLi-ion battery, and further the understanding on the Li storageability/mechanisms inside the hybrid structures.

The synthesized nanoporous graphene/Fe₂O₃ hybrids as anodes have thefollowing advantageous characteristics: (1) carbon-black- andbinder-free. Although graphene has been widely reported as conductivebackbone for metal oxides, carbon-black- and binder-free electrodes arerare. (2) Scalability. The nanoporous graphene/Fe₂O₃ hybrids areready-to-use freestanding nanoporous films, with an impressive thicknessof ˜250 μm—which is at least one order of magnitude thicker than anygraphene/metal oxides electrodes reported so far. No further processingor mixing is needed. This advantage is a product of the GMA scaffold,which has a large fraction of nanometer-sized pores and an open porestructure, and thus allows the electrolyte in direct contact with mostFe₂O₃ nanoparticles in all portions of the electrode. The small poresphysically limit the growth of nanoparticles, significantly shorteningthe Li ion diffusion pathway in the anode. In addition, nanopores forcemultiple sides of nanoparticles in close contact with graphene sheets(see FIG. 1), leading to tented nanoparticle structures that facilitateefficient electron transport to the conductive electrode. As such, themeasured specific capacity of the nanoporous graphene/Fe₂O₃ hybrids isessentially thickness independent. This is in contrast to othercarbon-black- and binder-free nanoparticle films, which show fastcapacity fading as the thickness increases by only a few micrometers (Haet al., Nano Lett. 2012, 12 (10), 5122-5130). (3) High surface area,high electrical conductivity, and mechanical robustness. Theseproperties are attributed to the 3D construction which uses conductiveand strong carbon cross-links between graphene sheets and the remarkableproperties of individual graphene sheets. This combination of traits isnot readily achievable in many other nanoporous materials, which oftenhave low electrical conductivity, low strength, or closed pores, andthus are not suitable for electrode applications.

Example 2 Graphene-TiO₂ Monolith

The TiO₂ sol-gel was prepared via a two-step process involvingacid-catalyzed hydrolysis of titanium (IV) ethoxide (1 g) using water(85.7 ml), hydrochloric acid (37%, 71.4 ml), and ethanol (3.57 g),followed by base-initiated gelation using propylene oxide (0.357 g).Composites were synthesized by infiltration of graphene macroassemblies(GMA) by the oxide sol-gel solutions prior to gelation. The graphenemonoliths were immersed in the sol-gel solutions and placed under vacuumuntil no more air escaped from the scaffolds, indicating fullpenetration of the sol. The concentration of inorganic precursors waskept low to promote the growth of the condensed inorganic phaseprimarily on the surfaces of the 3D graphene framework, while minimizinggelation in the free pore volume of the aerogel. The infiltratedgraphene aerogels were then cured at room temperature for 72 h toproduce the wet oxide/GMA gels. The wet oxide/GMA gels were dried usingsupercritical extraction with liquid CO₂ to yield the final dryoxide/GMA composites Annealing at 320C in air for 5 hours was used toconvert the TiO₂ to the anatase crystalline phase.

FIGS. 8-11 show x-ray, Raman, and electrochemical characterization.

After annealing, amorphous TiO₂ transit into anatase phase with grainsize of ˜3 nm.

Li storage capacity decreased from 156 mAh/g to 110 mAh/g in the firstdelithiation process.

The coulombic efficiencies were similar.

While not limited by theory, the decrease of capacity may be attributedto the oxygen deficiency in the annealed sample.

Example 3 Graphene-SiO₂ Monolith

The graphene macroassembly (GMA) scaffolds were prepared as described inWorsley et al., Chem. Commun. 48:8428-8430 (2012). The oxide/GMAcomposites were prepared through deposition of an oxide coating over theinner surface area of the GMA framework using sol-gel chemistry. TheSiO₂ sol-gel was prepared via traditional one-step base-catalyzedalkoxide sol-gel chemistry using tetramethoxysilane (4.1 g), water (1.5g), ammonium hydroxide (30%, 200 ml), and methanol (24 g). See Iler, R.K., The Chemistry of Silica. 1979, New York: John Wiley & Sons. 896,incorporated herein by reference in its entirety.

Composites were synthesized by infiltration of GMA monoliths by theoxide sol-gel solutions prior to gelation. The GMA's were immersed inthe sol-gel solutions and placed under vacuum until no more air escapedfrom the scaffolds, indicating full penetration of the sol. Theconcentration of inorganic precursors was kept low to promote the growthof the condensed inorganic phase primarily on the surfaces of the GMAframework, while minimizing gelation in the free pore volume of theaerogel. The infiltrated GMAs were then cured at room temperature for 72h to produce the wet oxide/GMA gels. The wet oxide/GMA gels were driedusing supercritical extraction with liquid CO₂ and annealed to yield thefinal dry oxide/GMA composites.

Example 4 Graphene-SnO₂ Monolith

The graphene macroassembly (GMA) scaffolds were prepared as described inWorsley et al., Chem. Commun. 48:8428-8430 (2012). The oxide/GMAcomposites were prepared through deposition of an oxide coating over theinner surface area of the GMA framework using sol-gel chemistry. TheSnO₂ sol-gel was prepared via an epoxide-initiated gelation method usingtin chloride pentahydrate (0.56 g), trimethylene oxide (1.03 g), ethanol(7 g), and water (5 g). See Baumann et al., Advanced Materials17(12):1546-1548 (2005), incorporated herein by reference in itsentirety.

Composites were synthesized by infiltration of GMA monoliths by theoxide sol-gel solutions prior to gelation. The GMA's were immersed inthe sol-gel solutions and placed under vacuum until no more air escapedfrom the scaffolds, indicating full penetration of the sol. Theconcentration of inorganic precursors was kept low to promote the growthof the condensed inorganic phase primarily on the surfaces of the GMAframework, while minimizing gelation in the free pore volume of theaerogel. The infiltrated GMAs were then cured at room temperature for 72h to produce the wet oxide/GMA gels. The wet oxide/GMA gels were driedusing supercritical extraction with liquid CO₂ and annealed to yield thefinal dry oxide/GMA composites.

What is claimed is:
 1. A composition comprising at least onegraphene-supported metal oxide monolith, said monolith comprising (i) athree-dimensional network of graphene sheets crosslinked by covalentcarbon bonds, and (ii) at least one metal oxide embedded inside saidthree-dimensional network, wherein the graphene-supported metal oxidemonolith is mesoporous, wherein the graphene-supported metal oxidemonolith has a surface area of at least 500 m²/g, wherein the metaloxide accounts for 40-80 wt. % of the graphene-supported metal oxidemonolith, and wherein the metal oxide comprises manganese, iron, cobalt,nickel, copper, zinc, zirconium, tin, silicon, aluminum, chromium,vanadium, titanium, or combinations thereof.
 2. The composition of claim1, wherein the graphene sheets are randomly oriented.
 3. The compositionof claim 1, wherein the covalent carbon bonds for crosslinking thegraphene sheets are primarily sp² bonds.
 4. The composition of claim 1,wherein the monolith has a thickness of 100 μm or more.
 5. Thecomposition of claim 1, wherein the metal oxide is in particle form andhas an average particle diameter of 50 nm or less.
 6. The composition ofclaim 1, wherein the metal oxide is Fe₂O₃.
 7. The composition of claim1, wherein the metal oxide is TiO₂, SiO₂, or SnO₂.
 8. An electrodecomprising the composition of claim 1, wherein the electrode issubstantially free of any carbon-black and substantially free of anybinder.
 9. An electrode comprising the composition of claim
 1. 10. Adevice comprising the electrode of claim
 9. 11. The device of claim 10,wherein the monolith is incorporated in an electrode, and wherein thedevice further comprising at least one counter electrode, at least oneelectrolyte, at least one separator, and at least one current collector.12. The device of claim 10, wherein the monolith is incorporated in anelectrode, wherein the electrode has a thickness of 100 μm or more, andwherein the electrode is substantially free of any carbon-black andsubstantially free of any binder.
 13. The device of claim 10, whereinthe metal oxide is Fe₂O₃, and wherein the device is a lithium ionbattery having a discharge capacity of at least 500 mAh/g at 100 mA/gafter 30 cycles.
 14. The device of claim 10, wherein the metal oxide isTiO₂, and wherein the device is a lithium ion battery having a dischargecapacity of at least 100 mAh/g at 168 mA/g.
 15. The device of claim 10,wherein the device is a battery, a capacitor, or a sensor.
 16. Thedevice of claim 10, wherein the graphene sheets are coated by at leastone particulate metal oxide.
 17. The device of claim 10, wherein thegraphene sheets are coated by at least one nanoparticulate metal oxide.